This invention relates to a radiation image storage panel having a stimulable phosphor layer, and in particular, a radiation image storage panel that can provide radiation images which are high in radiation sensitivity and sharpness.
Radiation images like X-ray images are often used in the diagnosis of diseases. Conventional X-ray image storage methods include those in which images are directly taken from a phosphor layer rather than utilizing a light-sensitive silver halide material. For example, radiation (generally X-ray) transmitted through a subject is absorbed by a phosphor, and thereafter this phosphor is excited by light or heat energy to bring the absorbed radiation energy stored to radiate as fluorescence. The fluorescence is detected and formed into an image.
Specifically, U.S. Pat. No. 3,859,527 and Japanese Unexamined Patent Publication No. 12144/1980 disclose radiation image storage methods in which a stimulable phosphor is used and visible light or infrared rays are used as stimulating light. This method employs a radiation image storage panel (hereinafter referred to as "storage panel") comprising a support formed thereon with a stimulable phosphor layer (hereinafter referred to simply as "stimulable layer"). Radiation transmitted through a subject is absorbed by the stimulable layer and radiation energy corresponding to the radiation transmission degree of all areas of the subject is stored to form a latent image. Thereafter this stimulable layer is scanned with the stimulating light causing the stored radiation energy to into light. Thus, an image according to signals based on the strength of this light is obtained.
The image finally obtained may be reproduced as a hard copy, or may be reproduced on a CRT.
Generally speaking, the radiation sensitivity of the storage panel has a tendency to be higher when the stimulable layer is thick. On the other hand, the sharpness of the storage panel has a tendency to be higher when the thickness of the stimulable layer is decreased.
Prior art concerning the storage panel have been disclosed in, for example, Japanese Unexamined Patent Publication No. 11393/1981 in which a metal light-reflective layer is provided on one intersurface of a stimulable layer. The stimulable layer is prepared by dispersing stimulable phosphors into binders. According to this method, the metal light-reflective layer replaces the inner part of the stimulable layer that is away from a surface of the stimulable layer to which the stimulating light incidents. The stimulable layer can be made thinner, and the spread of the stimulating light into the stimulable layer can be suppressed. Therefore, a radiation image with high sharpness is obatined. Although this method can suppress the spread or scattering of the stimulating light in the layer because of the decrease in thickness of the stimulable layer, the stimulating light to reach the metal light-reflective layer while scattering in the layer has poor directivity. The stimulating light is reflected corresponding to the incidence with the metal light-reflective layer and returned to the stimulable layer side to repeat scattering in the stimulable layer again. The stimulable phosphor is widely stimulated, thus the improvement of sharpness of images is low.
Japanese Unexamined Patent Publication No. 12600/1981 discloses a method in which a reflective layer of white pigments is provided (instead of the metal light-reflective layer as described in Japanese Unexamined Patent Publication No. 11393/1981) on one surface of a stimulable layer, which is formed by dispersing stimulable phosphors into binders. According to this method the light-reflective layer of white pigments replaces the inner part of the stimulable layer that is away from a surface of the stimulable layer to which the stimulating light incidents. Thus, the thickness of the stimulable layer can be further decreased to enable the suppression of the spread of the stimulating light into the stimulable layer, resulting in the production of radiation images with high sharpness.
However, the stimulable phosphor is a kind of white pigment. That is, this method is conducted by merely replacing a part of the stimulable layer, which has been formed by dispersing the stimulable phosphor into the binders with the white pigment layer which is formed by dispersing the white pigment into the binders. For this reason, this method can suppress the spread or scattering of the stimulating light in the stimulable layer with decreased thickness of the stimulable layer. However, the stimulating light that reaches the light-reflective layer of white pigment while scattering in the stimulable layer is reflected irregularly on the surface of the light-reflective layer of white pigments, or scattered in the light-reflective layer of white pigments and reflected to the stimulable layer side. Thus, the stimulating light is scattered in the stimulable layer again to stimulate the stimulable phosphor widely, resulting in less improvement of sharpness of images.
A stimulable layer containing no binder, as described in Japanese Unexamined Patent Publication No. 73100/1986, can significantly improve not only the charge ratio of the phosphor, but also the directivity of the stimulating light and stimulated emission in the stimulable layer. This results in an improvement of the sensitivity of the storage panel to radiation and, at the same time, an improvement in the sharpness of images. Since the vapor deposition and sputtering methods are appropriate for the preparation of the stimulable layer containing no binder, the support used must have heat-resistance. For this reason, crystallized glasses, chemically reinforced glasses and the like can preferably be used as a support. However, these supports also must be somewhat thick. Thus, a part of the stimulating light is scattered violently in the support, resulting in less sharp images.
The present inventors have proposed a storage panel in which a light-reflective layer is provided on an intersurface of either one side of the stimulable layer, described in Japanese Unexamined Patent Publication No. 133399/1987, and a storage panel in which a light-scattering layer is provided on an intersurface of either one side of the stimulable layer, described in Japanese Unexamined Patent Publication No. 133400/1987. Although these storage panels have excellent radiation image sensitivity and sharpness of images, there is a room for improvement.